Abstract
Abstract. To investigate the sources and evolution of haze pollution in different seasons, long-term (from 15 August to 4 December 2015) variations in chemical composition of PM1 were characterized in Beijing, China. Positive matrix factorization (PMF) analysis with a multi-linear engine (ME-2) resolved three primary and two secondary organic aerosol (OA) sources, including hydrocarbon-like OA (HOA), cooking OA (COA), coal combustion OA (CCOA), local secondary OA (LSOA) and regional SOA (RSOA). The sulfate source region analysis implies that sulfate was mainly transported at a large regional scale in late summer, while local and/or nearby sulfate formation may be more important in winter. Meanwhile, distinctly different correlations between sulfate and RSOA or LSOA (i.e., better correlation with RSOA in late summer, similar correlations with RSOA and LSOA in autumn, and close correlation with LSOA in early winter) confirmed the regional characteristic of RSOA and local property of LSOA. Secondary aerosol species including secondary inorganic aerosol (SIA – sulfate, nitrate, and ammonium) and SOA (LSOA and RSOA) dominated PM1 during all three seasons. In particular, SOA contributed 46 % to total PM1 (with 31 % as RSOA) in late summer, whereas SIA contributed 41 % and 45 % to total PM1 in autumn and early winter, respectively. Enhanced contributions of secondary species (66 %–76 % of PM1) were also observed in pollution episodes during all three seasons, further emphasizing the importance of secondary formation processes in haze pollution in Beijing. Combining chemical composition and meteorological data, our analyses suggest that both photochemical oxidation and aqueous-phase processing played important roles in SOA formation during all three seasons, while for sulfate formation, gas-phase photochemical oxidation was the major pathway in late summer, aqueous-phase reactions were more responsible during early winter and both processes had contributions during autumn.
Highlights
Considering that the long-term measurements in our study have different meteorological conditions, we separated the entire study into three periods as late summer (15 August to 10 September), autumn (11 September to 10 November) and early winter (11 November to 4 December) in order to discuss the seasonal variations in PM1 mass concentration and chemical composition
An aerosol chemical speciation monitor (ACSM) combined with an aethalometer were applied for real-time measurements of PM1 species from 15 August to 4 December 2015 in Beijing
The average mass concentration of PM1 varied from 21.6 in late summer to 64.3 μg m−3 in early winter, indicating that particulate matter (PM) pollution was very serious in wintertime due to enhanced emissions, low temperatures and stagnant meteorological conditions
Summary
Atmospheric particulate matter (PM) has broad impacts on the environment, including air quality (Molina et al, 2007; Sun et al, 2010, 2013; Huang et al, 2014), regional and global climate (Kaufman et al, 2002; IPCC, 2013; Molina et al, 2015), and human health (Pope et al, 2002; Lelieveld et al, 2015). In combination with the back-trajectory analysis, it is found that high PM1 concentrations in Beijing were associated with air masses from the south and southwest and characterized by high fractions of MO-OOA and secondary inorganic aerosol, whereas direct emissions from local sources were the main contributor during clean events (Sun et al, 2015). These results show the inhomogeneity in the contribution to PM pollution depending on different sampling locations and seasons, highlighting the need for more studies on chemical composition, sources and atmospheric evolution of PM. The formation mechanisms of secondary species and the impacts of meteorological conditions on the haze pollution are elucidated
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